PT2069547E - Steel composition for special uses, in particular in the automotive field - Google Patents

Abstract

The invention relates to a steel composition for special uses, characterized by the following contents where the percentages are expressed by weight: Carbon 0.04 to 0.20%, Silicon 1.00 to 2.50%, Chromium 16.0 to 25.0%, Nickel 8.0 to 15.0%, Nitrogen 0.10 to 0.20%, Cerium 0.03 to 0.08% and characterized by high resistance to high temperature fatigue. This composition is most particularly suited to making automotive industry parts, such as for example features or components of a recycling circuit for exhaust gases (EGR circuit) for a thermal motor in an automotive vehicle.

Description

1

DESCRIPTION " STEEL COMPOSITIONS, FOR SPECIAL APPLICATIONS, IN PARTICULAR IN THE AUTOMOBILE SECTOR " The invention relates to special steels and relates more particularly to the use of a steel composition, especially in the automotive sector.

There are numerous steel compositions which generally correspond to " bellows " whose contents in its main constituent elements are defined by norms.

Industrialists are constantly on the lookout for steel compositions to meet specific needs where particularly heavy constraints prevail. It is in this context that the Applicant has presented the problem of searching for steel compositions and therefore parts obtainable therefrom which are capable of meeting the new environmental and heat stress resistance constraints encountered in particular in automotive applications . The above problem has two aspects, on the one hand, the restrictions on the emission of exhaust gases, and on the other hand the restrictions linked to the problems of life and end of life of motor vehicles, their deconstruction and the recycling of its components.

It will be recalled in this respect that the European Community specified in its Directive 70/220 / EEC limits 2 taxes on new light vehicles (category Ml and Nl), that is to say a standard emission level " (g / km) refer to the following emissions: CO, HC, HC + NOx, NOx, PM. The term " PM " is the abbreviation for the Anglo-Saxon expression " Particulate Material " which means " particular material ".

These values have been regularly reviewed down to the extent of their updating, whose stages in time were or will be: - in July 1992 the Euro 1 standard - in January 1996 the Euro 2, IDI and Dl - in January 2000 the Euro 3 standard - in January 2005 the Euro 4 standard - and in mid-2008 the Euro 5 standard

On the other hand, the restrictions on the end of life of vehicles are defined in the European Directive 2000/53 / EC of 18 September 2000 and also present in vehicles vehicles M1 and N1. This Directive insists inter alia on the obligation for car manufacturers to seek and apply solutions that allow optimum recycling of the different components.

These different restrictions have consequences on the characteristics of the vehicles and more particularly on the technical characteristics of the engines and their different components.

Thus, the first restrictions forced motor vehicle manufacturers to reduce engine consumption by maintaining, even increasing, their power in order to reach known values for 3 larger displacement engines in accordance with the previous concepts.

This objective could be achieved by combining two factors, namely the reduction of engine displacement and the increase in operating pressure as a corollary to an increase in the operating temperature, allowing the whole to improve the thermal performance of these new engines engines.

A consequence of this increase in operating temperature is the increase in the temperature of the exhaust gases, which went from about 400 ° C to about 700 ° C, even further.

On the other hand, in order to reduce in this type of engine the emission values because of first of the " unburned ", has been adapted an exhaust gas recycling system which are injected back into the engine feed circuit .

The exhaust gases are thus re-injected, or replaced in circulation, by an " exhaust gas recycling system " also called " Exhaust Gas Recycling " (Anglo-Saxon term known by its abbreviation EGR), being this system constituted by several components, whose tubes are of steel.

Such recycling systems are components subjected to particularly high stresses. It is especially the case of the tube referred to " EGR tube " which carries the exhaust gas into the heat exchanger which serves to cool the exhaust gases prior to their injection into the engine. This tube is in effect traversed by exhaust gases whose temperature can reach and exceed 700 ° C. The heat exchanger comprises a tubular element called " refrigerator " (" cooler ") which is passed through a coolant whose temperature in normal operation is usually in the range of 80 to 100 ° C.

On the other hand, environmental restrictions on the life of vehicles and their engines, as well as restrictions on deconstruction of vehicles have led manufacturers to look for particularly strong materials. These materials must withstand the corrosion that can be caused by the gases and other corrosive components that result from the combustion of the fuels and also by the condensates that form when the engine is stopped. They should furthermore have a good resistance to hot fatigue due to the mechanical and vibratory pressures to which these engine environment elements are subject. But these materials must also have characteristics of resistance in time allowing them, both at the time of the replacement of parts and at the time of deconstruction of vehicles, to recover used or missing components that can be recycled.

In addition, the pipes used to manufacture the elements to be assembled to form such equipment must be capable of withstanding cold deformations and welding operations at the time of production and assembly of the elements in order to constitute the assembly called EGR. The materials constituting these elements must therefore be capable of withstanding stretching of at least 40% without damage by their subsequent use. 5

For these different reasons builders are looking for solutions that apply metals or their alloys that have good corrosion resistance and good resistance to hot fatigue and which can be recycled.

With regard to firmness (or resistance) to fatigue, a minimum threshold to be exceeded, serving quite a current of significant value in the mechanical engineering sector, and in particular motor vehicle construction, is the number of 1 000 000 cycles.

In other words, motor vehicle manufacturers, when characterized by fatigue strength tests, perform the performance of components intended for the construction of vehicles, are often led to discard technical solutions which do not meet and exceed this minimum number of cycles under certain conditions or restrictions for each type of components. It is in this context that the Applicant sought solutions from known steel compositions and whose properties had not been fully exploited. The prior art is illustrated by the following documents. - FR-A-1-2 283 238 (AVESTA JERNVERKS AB [SE]) March 26, 1976 (1976-03-26) - US-A-4-224 062 (DARNFORS SVEN) September 23, 1980 -09-23) - http://www.worldstainless.org/NR/rdonlyres/1563541 A-23AD-4869-A5BF-D4FlFC963257 / 2415 / StainlessSteelApplicationsAutomotive.pdf Extract from the Internet: Url: http: //www.worldstainless. org / About + st 6 ainless / What + can / Transportation / Automotive + Applications / Document s + Listing.htm > - Extract from the Internet: URL: www.outkumpu.com > - ANDERSSON ET AL: " Low cycle fatigue of four stainless steels in 20% CO-80% H2 " INTERNATIONAL JOURNAL OF FATIGUE, BUTTERWORTH SCIENTIFIC LTD, GUILDFORD, GB, vol. 29, no. 1, January 2007 (2007-01), pages 119-127, ISSN: 0142-1123 - SANDSTROEM R ET AL: " Elevated temperature low cycle fatigue of the austenitic stainless steels type 316 and 253 MA-influence of microstructure and damage mechanisms " HIGH TEMPERATURE TECHNOLOGY SCIENTIFIC LTD, GUILFORD, UK, vol. 7, 1989, pages 2-10, LINDE L ET AL: " Thermo-mechanical and low cycle fatigue behavior of the austenitic stainless steel 253 MA " SWEDISCH INSTITUT FOR METAL RESEARCH, vol. IM-1999-15, April 1999 (1999-04), pages 1-28,

These documents refer to steel compositions for special applications in the automotive sector. However none of these documents describes or suggests the use of such compositions for the manufacture of an exhaust gas recycling circuit (EGR circuit) under conditions of high resistance to hot fatigue as defined below. The invention relates more particularly to the use of a steel composition having the following percentages by weight: Carbon: 0.04 to 0.20% Silicon: 1.00 to 2.50% Chromium: 16 , 0 to 25.0% Nickel: 8.0 to 15.0% Nitrogen: 0.10 to 0.20% Cerium: 0.03 to 0.08% 7 and the following additional contents:

Manganese: 1,00% maximum Phosphorus: 0,045% maximum

Sulfur: 0,030% maximum, the complement consisting of iron and impurities, for the manufacture of an exhaust gas recycling tube (EGR circuit) in which the tube has a high resistance to hot fatigue, representing in number of cycles in excess of 1,000,000 cycles, at temperatures of at least 700 ° C, under tensions of 300 MPa in the annealed state and 800 MPa in the hardening state.

In fact, samples obtained from a composition according to the invention were tested and allowed to place portions of the Wöhler curves according to the cases at temperatures of 20 ° C, 400 ° C and 700 ° C under values of maximum tensions that are between 250 and 700 MPa in the annealing state and between 600 and 1200 MPa in the hardening state.

These strokes expose a hot fatigue (700 ° C) resistance of a minimum of 1 000 000 cycles under maximum stress of 300 MPa in the annealed state and 800 MPa in the hardening state. In the context of the invention, a figure of 1 000 000 cycles is also retained as a significant value of resistance to hot fatigue, a value widely used in mechanical engineering and in particular in the automotive sector as the minimum threshold to be attained.

In the composition of the invention, iron is evidently the predominant constituent. However, in addition to the contents mentioned above, the composition may comprise other minor constituents, for example in the trace and also in any impurities.

Different tests performed on steel compositions responding to the above characteristics have shown that these compositions exhibited especially important hot fatigue resistance properties at temperatures of greater than or equal to 700 ° C.

Such steel compositions which are austenitic type stainless steel compositions allow to realize components with specific shapes, in particular tubular components, by different known forming processes, in particular by hydroforming. Such compositions are therefore particularly well suited for making components for EGR systems as defined above.

In a preferred embodiment of the invention, the steel composition comprises the following amounts:

Carbon: Silicon: Chrome: Nickel: Nitrogen: Cerium: The composition close. 0.04 to 0.20% 1.00 to 2.50% 18.0 to 22.0% 9.0 to 12.0% 0.10 to 0.20% 0.03 to 0.08% of the invention comprises two subfamilies 9

In a first sub-family, the composition of the invention comprises the following contents:

Carbon: 0.04 to 0.08% Silicon: 1.00 to 2.00% Chromium: 18.0 to 20.0% Nickel: 9.0 to 11.0% Nitrogen: 0.12 to 0.20% Cerium: 0.03 to 0.08%

This first subfamily includes a known tone which is characterized by supplemental levels of manganese, phosphorus and sulfur. This is the 1.4818 tone according to the NF EN standard or S30415 according to ASTM.

In a second sub-family, the composition of the invention comprises the following contents:

Carbon: 0.05 to 0.12% Silicon: 1.40 to 2.50% Chromium: 20.0 to 22.0% Nickel: 10.0 to 12.0% Nitrogen: 0.12 to 0.20% Cerium: 0.03 to 0.08%

This second subfamily includes a tone that is characterized by supplemental levels of manganese, phosphorus and sulfur. It is the 1.4835 tone according to the NF EN standard or even S30815 according to ASTM.

In use according to the invention, the pipe is a straight pipe which can be obtained by longitudinal welding of a strip of metal, or web, according to methods well known to man of the art for the manufacture of welded pipes. The tube may also be obtained by rolling, in particular a hot rolling process, followed if necessary by a cold rolling, according to the dimensional and mechanical characteristics required for its use or its subsequent transformations. It is also possible to realize the tube by extrusion or drawing. These processes are also well known to man in the art.

In another aspect, the invention relates to an exhaust gas recycling (or recirculating) circuit tube (EGR circuit) for a motor vehicle thermal engine obtained by transforming a pipe as defined above.

For the realization of such a tube, or part of a right tubular element of chosen dimensions to which processing operations in general are subjected in cold. Such transformation operations may for example comprise steps of forming circular undulations (also called " bellows ") by hydroforming, bending, widening or any other operation in order to obtain the tube according to the plane as defined by its creator. The EGR tube can be obtained either in the annealed state or in the untreated state after cold deformation, depending on the application concerned. In the case of EGR circuits, the tubes obtained may be thermally treated prior to assembly if the conditions of their deformation lead to the need for heat treatment in order to enable them to meet their subsequent conditions of use. 11

In the following description, given by way of example only, the attached drawings relate to: FIG. 1 shows schematically a tensile / compression machine used for testing test tubes made from compositions according to the invention; invention and from reference gauges for comparison purposes; Figure 2 shows four WÖHLER curves obtained from tests performed on the machine of figure 1 and respectively on parts made in the four steel compositions which have known applications in sectors such as steelworks, cement works, the manufacture of parts for kilns , the tests being conducted at an ambient temperature (20 ° C) and on samples in the annealed state; Figure 3 shows four WÖHLER curves obtained under conditions analogous to those in figure 2, unless the four test tubes are in the cold worked state; Figure 4 takes up two of the curves of Figure 2 and shows measurement points from tests performed at 400 ° C and 700 ° C for two samples in the annealed state; figure 5 takes up two of the curves of figure 3 and shows measurement points from tests carried out at 400 ° C and 700 ° C by two samples in the cold worked state; and Figure 6 represents a component of an EGR system made in a steel composition according to the invention.

A number of tests will now be described which have been carried out and which have made it possible to highlight the properties of steel compositions which correspond to the definition mentioned above. The purpose was to test known tones in applications which require a hot fatigue resistance but under less demanding conditions of stress (stresses, vibrations, temperature) or which may exhibit features that are at least partially responsive to the characteristics being sought.

The tests were based on five tones referenced according to their designations mentioned in the ASTM and NF EN standards and, for some of them, the indication of their current denomination. These references and denominations are shown in Table 1 below:

Table 1

Designation ASTM Standard Standard NF (EN designation) EN 304L S30403 1.4307 309S S30908 1.4833 321 S32100 1.4541 S30415 1.4818 S30815 1.4835

The first three shades were tested for known applications in sectors such as steelmaking, cement making, furnace parts manufacturing.

The last two shades are known for their refractory characteristics, and it seemed interesting to the applicant to know their fatigue resistance characteristics. 13

Few technical elements are available in the bibliographies, publications, etc., on the characterization of the steels of resistance to hot fatigue.

Both the fatigue strength characterization method of steels is known under their title " Fatigue tests " and described in " Engineer Techniques, Treated Metal Materials " (reference M4 170), as the publications on tests of resistance to hot fatigue are in limited numbers.

These publications include an article published in Steel Research 60 (1989) No. 8 where it will be noted that the materials tested were as follows, as shown in Table 2 below:

Table 2

Designation Standard ASTM Standard standard (UNS designation) NF EN AISI 304 S30400 1.4301 AISI 409 1.4512 AISI 316 S31635 1.4571 Ti

For any of the tones tested by the applicant there were technical elements (tests, bibliographies, publications, etc ...) giving indications on the resistance to hot fatigue under conditions of tensions (stresses) and vibrations for the considered applications, especially for the EGR circuits.

The products tested were tubular components obtained according to the following manufacturing range. The first material is a band having a width of 79.7 mm and a thickness of 0.4 mm in accordance with EN 10 217.7.

These tubular components were obtained by the following transformation steps: - forming, TIG welding and induction line heat treatment (in an automatic welding line (950 ° C temperature per pass in an induction coil of a few seconds running at high speed of welding - 2 to 15m / mn) then cut in the air to obtain a tube with a diameter of 24.8 mm and a thickness of 0.4 mm - in-line finishing with non-destructive control followed by a cut in suitable segments for the realization of the final component followed by a visual control; - hydroforming a tube segment to form blow belts; - thermal treatment of the hydroforming component at 1100 ° C.

The components tested are raw hydroforming components, therefore in the hardening state, and thermally treated hydroforming components (also referred to annealed). The characterization is carried out by means of a repeated traction test with a load ratio R = 0.1 which is the ratio between the minimum traction and the maximum traction, for example minimum traction of + 10 daN and maximum traction of + 100 daN . 15

The fatigue tests are carried out: - at room temperature (20 ° C)

- at 400 ° C

- at 700 ° C and in two different states: - annealed components after hydroforming - raw hydroforming components. The machine used is a tensile / compression machine as shown schematically in Figure 1 of the accompanying drawings. It is a tensile / compression fatigue machine, the test frequency being 15 Hz. The sample tested is a tubular component 1 which includes a bellows-shaped central portion, the end 2 of which is engaged with a force sensor 3 and whose opposing end 4 is held in a take-off member 5 to which a force is applied as indicated by arrow F. The purpose of the tests was to always define the WÖHLER curve for each material, for each state and for each temperature.

It will be recalled that the WÖHLER curve is a resistance diagram obtained from a fatigue test consisting of subjecting a test tube to periodic, maximum amplitude and constant frequency cycles and recording the number of N cycles at the end of which the break occurs. This number N, usually on a logarithmic scale, is a function of the maximum stress (σ) of the cycles. Each point corresponds to a plane (σ, Ν), and from a batch of test tubes subjected to different maximum stresses 16, a curve is obtained which has a particular aspect.

In most cases, it is possible to trace an asymptotic branch that corresponds to the resistance limit or fatigue limit. The use of WÖHLER curves to determine the fatigue strength of certain stainless steels has already been described by Professor Erwin ROEDER in Steel Research 60 (1989) No.8, pages 375-382, but for steels other than those of the invention. Figure 2 shows four WÕHLER Ci, C2, C3, and C4 curves obtained by four steel compositions corresponding respectively to tones 304L, 309S and 321 according to the above-mentioned common denomination and 1.4818. In the example, the tests were conducted at room temperature (20 ° C) and on samples in the annealed state.

It will be appreciated that such curves each have a characteristic shape as mentioned above.

The curves of Figure 2 show that the composition according to tone 1.4818 (curve C4) exhibits superior performances to the other tones and in particular to tone 321 (curve C3) taken as reference in the automotive industry.

Analogous tests were conducted on five materials mentioned above at temperatures of 20 ° C, 400 ° C and 700 ° C, respectively in the annealed state (state A) and in the cold worked state (CW state) 3 shows four WÖHLER C5, Ce, C7 and Ce curves obtained by the four steel compositions of figure 2, corresponding respectively to tones 304L, 309S, 321, and 1.4818. In this example, the tests were conducted at room temperature (20 ° C) and on samples in the cold worked state.

The curves of Figure 3 also show that the composition according to tone 1.4818 (curve Cs) shows higher performances than the other tones and in particular the tone 321 (curve C7) taken as a reference. Figure 4 reproduces curves C3 and C4 of Figure 2 which correspond respectively to tones 321 and 1.4818 and thus result from tests conducted at room temperature (20 ° C) and on samples in the annealed state. 4 different points from measurements performed on tones 321 and 1.4818 were also presented from tests performed at 400 ° C and 700 ° C, always on samples in the annealed state. Figure 5 reproduces curves C7 and Cs of Figure 3 corresponding to tones 321 and 1.4818 and therefore result from tests conducted at ambient temperature (20 ° C) and on samples in the cold worked state. 5 different points from measurements performed on tones 321 and 1.4818 were shown in Figure 5 from tests performed at 400 ° C and 700 ° C on samples in the cold worked state,

The results of Figures 4 and 5 also show good performances of the 1.4818 tone, relative to the 321 tone, taken as reference, in tests performed at elevated temperatures (400 ° C and 700 ° C), both for samples in the annealed state and for samples in the cold worked state.

The results obtained from the five previously tested materials have shown, against all expectations, far superior performances to what was thought to be possible, on two tested materials, namely those corresponding to tones 1.4818 and 1.4835 according to with the standard NF EN. These results are summarized in Tables 3 and 4 below. Table 3 shows the hot fatigue strength expressed in number of cycles under a maximum tensile stress of 300 MPa for the five materials in the annealed state (state A), respectively at 20 ° C, 400 ° C and 700 ° C . Table 4 is analogous to Table 3, except that the maximum tensile stress is 800 MPa and that the five materials are in the cold worked state so hardened (CW state).

To facilitate understanding of the results, successive limits expressed in number of cycles were retained: 200,000, 500,000, 1,000,000 and 2,000,000 cycles. The indication given in the tables shows the level not reached or reached or substantially exceeded at the time of testing, with values below the indicated level being exceeded, except in cases where the indicated minimum value of 200 000 cycles could not be reached. 19

Table 3

State Tone according to standard NF EN 1.4307 1.4833 1.4541 1.4818 1.4835 At 20 ° C More than Less than Less than More than 2,000,000 1,000,000 1,000,000 1,000,000 2,000,000 At 400 ° C Less than Less than Less than More than More than 1,000,000 500,000 500,000 1,000,000 2,000,000 At 700 ° C Less Less than Less than More than 1,000,000 500,000 200,000 500,000 1,000,000

Table 4

State Tone as per NF standard EN 1.4307 1.4833 1.4541 1.4818 1.4835 CW 20 ° C More than Less than Less than More than 2,000,000 1,000,000 1,000,000 1,000,000 2,000,000 cw 400 ° C Less than Less than Less than More than More than 1,000,000 500,000 500,000 1,000,000 2,000,000 CW 700 ° C Less Less than Numbers of More than 1,000,000 500,000 200,000 500,000 1,000,000

From the analysis of the results shown in Tables 3 and 4 above, it can be seen that at tones 1.4818 and 1.4835 they offer overall performances above the three other tones tested, and in particular at temperatures of 400 ° C and 700 ° C, since the limit of 1 000 000 cycles is reached and even in most cases exceeded, while the performances of the three other tones are substantially much lower than the temperature of 700 ° C which is the most representative temperature of the conditions of use. On your

The five preceding materials are also known for their corrosion resistance conditions of use. They are also known to be able to withstand cold forming operations with minimum stretches of 40%. Figure 6 shows a component, or part, of an EGR system made from a steel composition according to the invention. This component is an EGR tube designated as a whole by reference 6. The tube 6 comprises a bellows-shaped central portion 7, in the arcuate example, comprised between two ends 8 and 9 terminated by respective collars 10 and 11. Such a tube presents a high resistance to hot fatigue and can be traversed by exhaust gases at temperatures that can reach, even exceed 700 ° C, being subjected to high vibrations. The presence of the bellows allows compensating for variations in temperature due to expansion. The part of figure 6 is only an example among other components which can be made from tubes of a composition according to the invention. In the case of an EGR tube, it can not only serve as a passage tube for transporting exhaust gases but also a heat exchange tube in contact with a cooling fluid.

Lisbon, May 17, 2012

Claims (11)

Use of a steel component characterized by having the following percentages by weight: Carbon: Silicon: Chrome: Nickel: Nitrogen: Cerium: 0.04 to 0.20% 1.00 to 2.50% 16.0 to 25.0% 8.0 to 15.0% 0.10 to 0.20% 0.03 to 0.08% and the following supplementary contents: Manganese: Phosphorus: Sulfur: 1.00% at most 0.045% at maximum of 0,030% at most, the complement consisting of iron and impurities, for the manufacture of an exhaust gas recycling tube (EGR circuit), characterized in that the tube has a high resistance to hot fatigue and is expressed in terms of number of cycles exceeding 1,000,000 cycles, at temperatures of at least 700 ° C, under stresses of 300 MPa in the annealed state and 800 MPa in the set state. Use of a steel composition according to claim 1, characterized in that the composition has the following contents: Carbon: 0.04 to 0.20% Silicon: 1.00 to 2.50% 2 Chromium: 18.0 to 22.0% Nickel: 9.0 to 12.0% Nitrogen: 0.10 to 0.20% Cerium: 0.03 to 0.08% Use of a steel composition according to one of Claims 1 and 2, characterized in that the composition has the following contents: Carbon: Silicon: Chromium: Nickel: Nitrogen: Cerium: 0.04 to 0.08% 1.00 to 2 , 0.00% 18.0 to 20.0% 9.0 to 11.0% 0.12 to 0.20% 0.03 to 0.08% Use of a steel composition according to one of Claims 1 and 2, characterized in that the composition has the following contents: Carbon: 0.05 to 0.12% Silicon: 1.40 to 2.50% Chromium: 20 , 0 to 22.0% Nickel: 10.0 12.0%, the Nitrogen: 0.12 to 20% Cerium: 0.03 to 0.08% Use of a steel composition according to one of claims 1 to 4, characterized in that the pipe is obtained by longitudinal welding of a strip of metal. 3 Use of a steel composition according to one of claims 1 to 4, characterized in that the tube is obtained by rolling. Use of a steel composition according to one of claims 1 to 4, characterized in that the pipe is extruded. Use of a steel composition according to one of claims 1 to 4, characterized in that the pipe is drawn. An exhaust gas recycling circuit (EGR circuit) pipe of a motor vehicle thermal engine obtained by transforming a pipe as defined in one of claims 1 to 8. A pipe according to claim 9, characterized in that the transformation comprises a hydroforming. Tube according to one of Claims 9 and 10, characterized in that it is obtained in the cold formed state. Lisbon, May 17, 2012